1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of the Related Art
In electronic devices, fan motors are employed in order to cool heat-generating components. When driving the fan motor, a motor drive circuit for performing so-called soft-switching may be used that a drive current of a motor coil is gradually changed so as to reduce noise in the fan motor, for example (see, e.g., Japanese Patent Laid-Open Publication No. 2005-287196). The motor drive circuit according to the above publication is a motor drive circuit of a PWM (Pulse Width Modulation) system and the motor coil is driven based on a PWM signal. In such a motor drive circuit, the drive current of the motor coil can be changed with the duty ratio of the PWM signal being changed (see Japanese Patent Laid-Open Publication No. 2005-287196).
In the motor drive circuit according to the above publication, a charging voltage of a capacitor and a triangle wave are compared, to generate the PWM signal. Therefore, in order to change the duty ratio of the PWM signal in a slow manner, it is necessary to change the charging voltage of the capacitor in a slow manner. Thus, such a motor drive circuit requires a large-capacity capacitor, for example.